What makes old beer taste bad? Why, it's the trans-iso-alpha acids, of course

Beer, for the most part, is not like wine—it does not improve with age. Quite the contrary, in fact. Old beer is a comparatively unpalatable shadow of its former self—skunky in odor, bitter in aftertaste.

So what happens between the brewery and the bottle opener to make long-in-the-tooth beer taste bad? A team of researchers from—where else?—Germany is on the case. A group from the Technical University of Munich and the Bitburger brewing company last month reported a comprehensive analysis of how beer becomes bitter over time in the Journal of Agricultural and Food Chemistry.

Beer is supposed to have a pleasant bitterness, thanks to the contribution of hops. But over time some of those bitter compounds degrade into less appealing substances that lend the aged beer a harsh, bitter aftertaste.

With mass spectrometry, the researchers tracked how those hop-derived compounds changed during storage at a variety of temperatures. They found that a family of compounds known as trans-iso-alpha acids underwent significant degradation into nasty, bitter by-products, including a compound called tricyclocohumol. The concentration of tricyclocohumol in pilsner beer, for instance, increased by nearly a factor of four after eight months of accelerated aging in a bottle at 28 degrees Celsius. Pilsner kept in a bottle at roughly room temperature (20 degrees C) for four years had even more tricyclocohumol—nearly six times the initial concentration.

The breakdown of trans-iso-alpha acids is temperature-dependent, meaning that beer ages faster in warm storage conditions, but it also appears to depend on the initial acidity of the beer. The researchers collected samples of 10 different pilsner brands to compare how the beer's pH value affected the aging process. Even though the beers were all comparable in acidity, ranging from pH 4.3 to 4.55, the slight differences had a measurable impact on how much the hop-derived compounds degraded into unwanted bitter compounds. The more acidic beers accumulated more tricyclocohumol during storage.

The key to producing a fresh-tasting beer, then, is to control its pH during the brewing process and to store it in a cool place once it has been bottled. As a consumer, the easiest thing to do is probably just to avoid old beer altogether. Who knows what kind of trans-iso-alpha acid degradation might be going on in those dusty bottles on your grocer's shelf?

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